Why do arrows work? Perceptual origin of motion caricature

Purpose. Search for the perceptual origin of arrow symbols to understand why they are effective in conveying the sense of movement in static drawings. Methods. Numerous visual demonstrations (shown during the talk) and some geometric calculations indicate that moving shapes appear distorted due to the following two factors: 1) a motion gradient induced by the spherical shape of the retina and 2) the perceptual distortion of moving shapes due to the motion gradient. Results. Notably, the field of distortion resembles an arrow shape, suggesting that arrow symbols are effective because they are the "caricatures" of perceptual distortions that occur inevitably while observing moving objects. This perceptual theory of arrows accounts for such properties of arrow symbols as greater velocity being conveyed by more acute heads and the greatest sense of movement being perceived when the heads are fixated. It also explains the particular eye-fixation dependence of the movements conveyed by the class of static motion cartoons which derives from the same perceptual distortion. Conclusions. This is an example of the visual system making use of informative signals even if they arise from perceptual distortions due to the particular way in which images are processed by the visual system. An important point is that these informative distortions seem to be perceptually exaggerated in order to make the content of the information more distinctive, like in caricatured faces.